The present invention relates to a cementious composition used for oil and gas well cementing and a method of cementing an oil or gas well. More particularly, the present invention relates to a cementious composition that exhibits improved resilience/toughness.
Hydraulic cement compositions are utilized for carrying out a variety of operations in oil, gas, and water wells, including, but not limiting to, completion operations, such as primary cementing, and remedial operations, such as squeeze cementing. Primary cementing involves the placement of a hydraulic cement composition into the annular space between the walls of a wellbore and the exterior of a pipe, such as a casing disposed therein. The cement composition is pumped into the annular space and allowed to set into an annular cement sheath therein whereby the pipe, e.g., casing, is bonded to the walls of the wellbore by the set cement. In addition to bonding the pipe to the walls of the wellbore, the set cement prevents the undesirable flow of fluids into and through the annular space.
Squeeze cementing techniques usually involve the method of preventing undesirable movement of oil, gas, or water through small holes or cracks in pipes disposed in the wellbore; holes, cracks, voids, or channels in the annular cement sheath between the pipe and the wellbore; or annular spaces between the cement sheath and the pipe or walls of the wellbore in permeable zones or fractures in subterranean formations. Such holes, cracks, etc., are plugged by squeezing hydraulic sealant compositions such as cement therein which harden and form impermeable plugs.
Over the years, the exploration for oil and gas wells has moved to more challenging environments, which involve high pressures, high temperatures, deep water, etc. These extreme conditions lead to many problems in connection with cement compositions heretofore used in either primary cementing or remedial operations such as squeeze cementing. For example, it is not uncommon for the formation or reservoir to subside as the hydrocarbon is withdrawn from the reservoir. Additionally, there can also be movement of the area surrounding the formation toward the wellbore due to the tectonic forces or due to its plasticity. Furthermore, during well construction and production, the cement in the annulus, particularly in harsh environments, is subjected to changes in pressure and temperature from operations such as swapping of wellbore fluid, perforation, stimulation, production, injection, and shutdown.
Thus, the combination of the extreme environmental conditions, moving reservoir, and operations typically performed on the well generate stresses in the annular cement sheath between the pipe, e.g., casing, and the wellbore. When the integrity of the cement sheath is compromised, several scenarios, all of which are undesirable, can result. In a worse-case scenario, the wellbore can collapse, shutting off production until remedial work is done, and in many cases, this solution is cost-prohibitive.
Even in the absence of complete wellbore collapse, loss of integrity of the cement sheath in the annulus can lead to other undesirable scenarios as for example low to negligible pressure on the wellhead; loss of hydrocarbon due to water in the producing zone and/or hydrocarbon entering a thief zone, etc.
Compositions useful in the present invention comprise a hydraulic cement, sufficient water to form a slurry, and an effective amount of a uniformly dispersed resilience/toughness enhancing additive comprising a thermoplastic block copolymer having a tensile strength from 500 to 4000 psi, a recoverable elongation of 200 to 1000 percent and temperature stability in the range of 30 to 500xc2x0 F.
According to a method of the present invention, a seal can be formed in a wellbore or in one or more subterranean formations penetrated by the wellbore by placing a composition comprising a mixture of a slurry of a hydraulic cement and an effective amount of a generally uniformly dispersed resilience/toughness enhancing additive comprising a thermoplastic block copolymer in one or more of the subterranean formations or in the wellbore at a desired location. Once in position, the hydraulic cement containing the resilience/toughness enhancing additive is permitted to set to form in the location a plug or seal, the plug or seal comprising a hardened cement containing a generally uniform dispersion of the resilience/toughness enhancing additive. The block copolymer has a tensile strength of from 500 to 4000 psi, recoverable elongation of 200 to 1000 percent and exhibits temperature stability in the range of 30 to 500xc2x0 F.